Funnily enough, the model is being verified by observed data...
And of course aircraft are transitting UK airspace - its open above FL200!
BoughtTheFarm, you're right - test flights need to be instrumented and done properly, covering the whole range of flight levels across all areas, rather than just a jolly around the eastern Atlantic at FL whatever which proves nothing. Even then, If you go fishing (test flight) you might not catch a fish (ash) , but it does not mean there are no fish (ash) in the sea. (air)